Zirconium chemistry

Titanium and zirconium chemistry has some unique features the chair conformation of the M2N4 metallocycle in the exobidentate complexes and endo-bidentate coordination. 

Structural aspects of zirconium chemistry. A. Clearfield, Rev. Pure Appl. Chem., 1964, 14, 91-108 (94). 

The migration of organic groups on metals to a-halogenated groups has been developed by Negishi and used by Whitby in zirconium chemistry [51], A similar type of migration in the case of zirconacyclopentadienes is also possible, as shown in Eq. 2.72, which affords zirconacyclohexadiene derivatives [52]. 

A variety of new ligand designs and ligand combinations were used in attempts to mimic some properties of the ubiquitous bent metallocene environment at the early metal centers consequently, some of these systems were used in the further development of butadiene zirconium chemistry. The pyridine based chelate zirconium dichloride complex 43 cleanly formed the butadiene complex 44 upon treatment with butadiene-magnesium. Its structure shows that the C4H6 is arranged perpendicular to the chelate ligand plane. Complex 44 inserts one equivalent of an alkene or alkyne to form the metallacyclic 7i-allyl system 4545 (Scheme 13). 

In this context, zirconium chemistry could open up new prospects for developing deoxygenative approaches to cyclopropanes. The Zr-assisted variant of the Kulinkovich reaction has been reported (Scheme 4, Eq. 1) [10]. Cyclopropanols were also formed, as by-products in addition to homoallylic alcohols, from aliphatic acid chlorides (Eq. 2) [ 10]. The conversion of acid chlorides (and also esters in several cases) into the corresponding homoallylic alcohols... 

There are several complete compilations of the literature concerning zirconium and hafnium that take the reader up to about 1960 62, 344, 420, 558). Since then several reviews of more limited scope have been published, one on the structural aspects of zirconium chemistry 116), and others on the separation of zirconium and hafnium 578), aqueous chemistry 234, 533), and ion-exchange properties of zirconium compounds 29). In general, the data in the present review are drawn from publications since 1960, although references to earlier work are included where necessary to complete the picture. 

Clearfield [64CLE] reviewed the structural aspects of zirconium chemistry. One of the important findings of this review was the substantial amount of evidence that was presented for the absence of the zirconyl (ZrO ) ionic structure in aqueous solution and in solids. Even in monoclinic zirconium oxide, Zr-O-Zr bonds are present rather than the Zr=0 double bond. Many compounds in both aqueous solution and the solid state contain hydroxo-zirconium bridges (see Section V.2.2). Clearfield presented evidence of the moiety [Zr4(0H)g(H20)i6] occurring in both aqueous solution and the solid state and argued that measurements by Zielen and Connick [56Z1E/CON] to determine the stability of polymeric zirconium hydrolysis species confirmed the existence of this species in aqueous solution. 

Titanium and zirconium chemistry is conveniently divided between simple complexes and those based on the metallocene imit Cp2M (Cp = CsHs = cyclopentadienyl). Most simple complexes are oligomeric, insoluble, and difficult to characterise, although alkyl titanium complexes such as X3TiR have found some use as non-basic Grignard equivalents. The dicyclopenta-dienyl metal moiety, Cp2M, renders complexes monomeric, soluble, and easily characterised by NMR spectroscopy, and thus many applications based on these systems have been devised. The most stable electronic configuration of titanocene and zirconocene complexes has only 16 electrons in the valence shell, not the 18 electrons common in most of the rest of the transition metal series. The empty orbital this leaves on the metal is crucial for reactivity. 

Organotitanium and -zirconium chemistry already has an established place in organic synthesis and many reactions are covered elsewhere in the Practical Approach series. Examples include reductive coupling of carbonyl compounds with low valent titanium to form 1,2-diols or alkenes methylenation of ester carbonyl groups with titanocene methylidene (Cp2Ti=CH2) zirconium-catalysed methylalumination of alkynes and hydrozirconation of alkynes and alkenes with the Schwartz reagent, Cp2ZrHCl. ... 

Zinc crosslinked acrylic polymer latices have been widely used in the manufacture of floor polishes. Recent interest has focused on zirconium chemistry, primarily ammonium zirconium carbonate, which forms a polymeric ionic bridge across the carboxyl groups. 

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